In some spark plugs, in order to enhance ignition performance, a noble metal tip of platinum, iridium, or the like is fixed by welding to the end of a center electrode or a ground electrode located on a side toward a spark gap. Recently, in order to reduce costs of the electrodes, strong demand has arisen to reduce the diameter and size of a noble metal tip. In order to implement a reduction in diameter and size of the noble metal tip, welding the noble metal tip directly to an electrode is not efficient. Thus, there is known a spark plug configured as follows. Japanese Patent Application Laid-Open (kokai) No. 2004-134209 and Japanese Patent Application Laid-Open (kokai) No. 2009-158408 discloses a spark plug, wherein, in place of a sole noble metal tip, there is provided a tip body (hereinafter, may be referred to as the first tip) 11, which corresponds to a first electrode member and is formed from Ni, etc., and a noble metal tip (hereinafter, may be referred to as the second tip) 21, which corresponds to a second electrode member and is formed separately in a size smaller than conventionally known, as illustrated in FIG. 10A. As shown in FIG. 10B, the tip body 11 and the noble metal tip 21 are positioned and welded together into a composite tip 31, which corresponds to an electrode composite. The composite tip 31 is welded via the tip body 11 to, for example, a ground electrode body formed at the forward end of a metallic shell of the spark plug (or welded to a center electrode body).
FIG. 11 shows an example of such a spark plug 41. The spark plug 41 has an insulator 43. A center electrode 71 is disposed in a forward end portion of an axial bore of the insulator 43, and a metallic shell 51 surrounds the insulator 43. A ground electrode 61 has one end joined to a forward end 52 of the metallic shell 51 and another end facing the forward end of the center electrode 71. The ground electrode 61 is configured such that the composite tip 31, which is formed by joining the first tip 11 and the second tip 21 together, is joined to a ground electrode body 60.
The noble metal tip (the second tip) 21 assumes the form of a very small circular columnar shape having an outside diameter of 1 mm or less (e.g., about 0.7 mm to 0.8 mm) and a height of about 0.5 mm. The joining surface of the mate tip body (the first tip) 11 to which an end surface 23 of the second tip 21 is to be welded; i.e., an end surface (a distal end surface) 13 of the tip body 11, also has a very small outside diameter of about 0.8 mm. A portion 15 of the mate first tip 11, that is to be joined to an electrode (the center electrode or the ground electrode), has a relatively large outside diameter. Accordingly, as shown in FIG. 10, the first tip usually has a concentrically stepped circular columnar structure having different diameters such that the base portion 15 having an end surface 12 to be joined to an electrode (the center electrode or the ground electrode) has a large diameter, whereas the end surface (the distal end surface) 13 to which the second tip 21 is to be welded has a small diameter.
Meanwhile, the end surface 23 of the second tip 21 is welded to the small-diameter distal end surface (the end surface) 13 of the first tip 11 conventionally in the following manner. For example, as shown in FIG. 10, while the first tip 11 is held by a chuck 81, the second tip 21 is positioned and disposed on and then welded to the first tip 11. In this case, the first tip 11 is held by chucking the outer circumferential surface of the large-diameter base portion 15 of the first tip 11. The end surface 23 of the second tip 21 is then concentrically positioned and placed on the end surface 13 of a small-diameter circular columnar portion 17 of the chucked first tip 11. The other end surface of the second tip 21 is pressed with a press pin (not shown). Under the pressed condition, the chuck 81 is rotated about its center axis C1, and the end surfaces to be joined of the two tips 11 and 21 are circumferentially laser-welded along their outer circumferences.
A collet chuck mechanism having a plurality of chuck claws (hereinafter, may be referred to merely as claws) 83 is usually used in the chuck 81. As shown in FIG. 12, the chuck 81 has the following configuration: when a single cylinder (not shown) is driven, claws 83 which are disposed orthogonal to a rod of the cylinder and, as viewed from the axial direction of the rod, usually at equal angular intervals (divided evenly into thirds) simultaneously move forward at the same speed in respective closing directions, thereby clamping the first tip 11. Thus, theoretically, the first tip 11 is fixed concentric with a reference center (reference center axis) C1 of a chuck surface 82.
However, when the first tip 11 is held, i.e., chucked, within the chuck 81, as exaggeratedly represented with the solid lines in FIG. 12, the first tip 11 is fixed in such a condition as to involve a positional deviation (eccentric error) Z, which is in many cases a very small amount, from a position concentric with the reference center (the reference center axis of the chuck) C1 of the chuck surface 82, as represented with the dashed circles in FIG. 12. That is, as shown in FIG. 12, the first tip 11 is fixed such that an actual center axis (may be called the center) C2 of the first tip 11 is eccentric to the reference center axis C1 of the chuck 81. This is for the following reason: regardless of a collet chuck, in a chuck having a plurality of claws, in view of the mechanism thereof, it is impossible to move the claws forward 100% simultaneously at the same speed over the same stroke in units of several-μm. Therefore, holding the first tip 11 with the chuck 81 involves a problem that, at a minimum, produces a positional deviation Z of about 0.025 mm from the reference center axis C1 of the chuck on one side.
Meanwhile, as shown in FIG. 13, when the second tip 21 is supplied and disposed in relation to the first tip 11 chucked under the condition that such a positional deviation Z is involved, since the preset reference center axis C1 of the chuck 81 is fixed, by means of controlling disposition of the second tip 21 with respect to the center axis C1, the positional error of the center axis of the second tip 21 with respect to the reference center axis C1 of the chuck 81 can be restrained to a negligibly small level (an error on the order of about 0.005 mm) as compared with an unavoidable error peculiar to the chuck mechanism. That is, by use of a supply means which employs a servomechanism or the like, the second tip 21 can be disposed with involvement of substantially no error; i.e., with high accuracy, with respect to the reference center axis C1 of the chuck 81. Therefore, in view of an error peculiar to the chuck mechanism which unavoidably arises at the time of chucking the first tip, a conventional manufacturing method needs to employ an eccentric error Z of at least about ±0.025 mm on one side as tolerance for the center runout (coaxiality) of the second tip in relation to the first tip.
In this regard, in order to improve performance of a spark plug, demand for improvement of dimensional accuracy associated with welding of the second tip to the first tip is becoming stronger and stronger. Specifically, a currently required tolerance on coaxiality (eccentric error) between the first and second tips is about 0.01 mm to 0.015 mm on one side. Thus, for a method in which the first tip is fixed with the above-mentioned chuck or the like, and the second tip is supplied and welded to the fixed first tip, difficulty is encountered in satisfying such a severe tolerance requirement for coaxial accuracy.
According to conceivable measures to overcome the above problem, after the second tip is supplied and disposed on the first tip held by a chuck, coaxiality (eccentricity) between the first and second tips is measured or detected through image processing or the like, and positional correction is performed for example, by shifting, according to the measured eccentricity (error), the second tip so as to be aligned with the center axis of the first tip. However, since such positional correction is performed after the second tip is supplied and disposed on the first tip, the end surfaces of the tips in contact with each other rub against each other, potentially resulting in the occurrence of a defect, such as scratches, on the end surfaces. Also, since the positional correction is performed after the second tip is supplied and disposed on the first tip, the number of steps increases. As a result, the efficiency in manufacturing a composite tip may drop, and in turn, spark plug productivity may drop. Furthermore, when, subsequent to the positional correction in which the second tip is positionally shifted so as to be coaxial with the first tip held by the chuck, welding is performed on the outer circumferential edges of the joining surfaces of the tips while the chuck is rotated, the center of rotation of the chuck is the reference center axis C1 of the chuck, whereas the actual center axes of the tips deviate by an error from the reference center axis C1. Thus, there also arises a problem that the distance between a laser welding apparatus and a region to be welded (laser radiation distance) varies with rotation of the chuck.
The above-mentioned problem is not limited to the case of manufacturing the composite tip, which corresponds to an electrode composite, formed by welding together the first tip (the tip body), which corresponds to the first electrode member, and the second tip (the noble metal tip), which corresponds to the second electrode member. In the spark plug 41 shown in FIG. 11, the center electrode 71 assumes the form of an electrode composite composed of a center electrode body 70, which corresponds to the first electrode member, and an electrode tip 77, which is welded to the forward end of the center electrode body 70 and corresponds to the second electrode member. Manufacturing the center electrode 71 in the form of such an electrode composite also involves the above-mentioned problem, for the following reason: even in manufacture of the center electrode 71, by use of an apparatus similar to that mentioned above, the center electrode body 70 is chucked; the electrode tip 77 is supplied and then positioned and disposed on the forward end of the center electrode body 70; and steps similar to those mentioned above are carried out. That is, manufacturing not only the above-mentioned composite tip 31 and the center electrode 71, but also an electrode composite formed through welding of the first electrode member and the second electrode member and adapted to form an electrode of a spark plug has involved a similar problem for a reason similar to that mentioned above.